To assess the potential reduction of small bowel volume receiving high-dose radiation by using kilovoltage X-ray cone beam computed tomography (CBCT) and quantized margin selection for adaptive bladder cancer treatment.Twenty bladder patients were planned conformally using a four-field, 15-mm uniform margin technique. Two additional planning target volumes (PTVs) were created using margins quantized to 5 and 10 mm in the superior direction only. CBCTs (approximately 8 scans/patient) were acquired during treatment. CBCT volumes were registered with CT planning scans to determine setup errors and to select the appropriate PTV of the day. Margin reduction in other directions was considered. Outlining of small bowel in every fraction is required to properly quantify the volume of small bowel spared from high doses. In the case of CBCT this is not always possible owing to artifacts created by small bowel movement and the presence of gas. A simpler method was adopted by considering the volume difference between PTVs created using uniform and adapted margins, which corresponds to the potential volume of small bowel sparing.The average small bowel volume that can be spared by this form of adaptive radiotherapy is 31 +/- 23 cm3 (+/-1 SD). The bladder for 1 patient was systematically smaller than the planning scan and hence demonstrated the largest average reduction of 76 cm3. The clinical target volume to PTV margins in other directions can be safely reduced to 10 mm except in the anterior direction where, like the superior direction, the bladder showed significant variation.Online CBCT-assisted plan selection based on quantized margins can significantly reduce the volume of small bowel receiving high doses for some bladder patients. CBCT allows the 15-mm margins used in some directions to be safely reduced to 10 mm.